Light emitting diodes (LEDs) fabricated with conjugated organic polymer layers have attracted attention due to their potential for use in display technology.
In the field of organic polymer-based LEDs it has been taught in the art to employ a relatively high work function metal as the anode; said high work function anode serving to inject holes into the otherwise filled π-band of the semiconducting, luminescent polymer. Relatively low work function metals are preferred as the cathode material; said low work function cathode serving to inject electrons into the otherwise empty π*-band of the semiconducting, luminescent polymer. The holes injected at the anode and the electrons injected at the cathode recombine radiatively within the active layer and light is emitted. The criteria for suitable electrodes are described in detail by I. D. Parker, J. Appl. Phys, 75, 1656 (1994).
Suitable relatively high work function metals for use as anode materials are transparent conducting thin films of indium/tin-oxide (H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl Phys. Lett. 58, 1982 (1991)).
Alternatively, thin films of conducting polymers such as polyaniline (see P. Snuth, A. J. Heeger, Y. Cao, J. Chiang and A. Andreatta, U.S. Pat. No. 5,470,505) can be used as demonstrated by G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature, 357, 477 (1992), by Y. Yang and A. J. Heeger, Appl. Phys. Lett 64, 1245 (1994) and U.S. Pat. No. 5,723,873, by Y. Yang, E. Westerweele, C. Zhang, P. Smith and A J. Heeger, J. Appl. Phys. 77, 694 (1995), by J. Gao, A. J. Heeger, J. Y Lee and C. Y Kim, Synth. Met., 82,221 (1996) and by Y. Cao, G. Yu, C. Zhang, R. Menon and A. J. Heeger, Appl. Phys. Lett. 70, 3191, (1997). Thin films of indium/tin-oxide and thin films of polyaniline in the emeraldine salt form with certain counterions (PANI(ES)) are preferred because, as transparent electrodes, both enable the emitted light from the LED to radiate from the device in useful levels. Using a layer of PANI(ES), or blends comprising PANI(ES), directly between the ITO and the light-emitting polymer layer, C. Zhang, G. Yu and Y. Cao (U.S. Pat. No. 5,798,170) demonstrated polymer LEDs having bilayer electrodes with long operating lifetimes.
Despite the advantages of using PANI(ES) in a bilayer electrode of polymer LEDs (as described in U.S. Pat. No. 5,798,170), the low electrical resisitivity typical of PANI(ES) inhibits the use of PANI(ES) in pixellated displays. For use in pixellated displays, the PANI(ES) layer should have a high electrical sheet resistance, otherwise lateral conduction causes cross-talk between neighboring pixels. The resulting inter-pixel current leakage significantly reduces the power efficiency and limits both the resolution and the clarity of the display.
Making the PANI sheet resistance higher in a bilayer electrode by reducing the film thickness is not a good option since thinner films give lower manufacturing yield caused by the formation of electrical shorts. This is demonstrated clearly in FIG. 1, which shows the fraction of “leaky” pixels in a 96×64 array vs. thickness of the PANI(ES) polyblend layer. Thus, to avoid shorts, it is necessary to use a relatively thick PANI(ES) layer with thickness of about 200 nm.
With a film thickness of 200 nm or greater, the electrical resistivity of the PANI(ES) layer should be greater than or equal to 104 ohm-cm to avoid crosstalk and inter-pixel current leakage. Values in excess of 105 ohm-cm are preferred. Even at 105 ohm-cm, there is some residual current leakage and consequently some reduction in device efficiency. Thus, values of approximately 106 ohm-cm are even more preferred. Values greater than 107 ohm-cm will lead to a significant voltage drop across the injection/buffer layer and therefore should be avoided. To achieve high resistivity PANI(ES) materials with resistivities in the desired range requires reformulation of the PANI(ES).
Thus, there is a need for a formulation of high resistivity PANI(ES) for use in high efficiency pixellated polymer emissive displays.